Postdeflection tube with all rare earth phosphors

ABSTRACT

A color cathode-ray tube of the postdeflection-focusing type (PDF) wherein the phosphor screen exhibits a linear change in brightness with changes in electron beam current density over the entire operating range of the tube. The preferred phosphor screen employs trivalent europium activated yttrium vanadate as the redemitting phosphor, trivalent terbium activated yttrium phosphate as the green-emitting phosphor and divalent europium activated strontium chlorophosphate as the blue-emitting phosphor. The disclosed screen results in a PDF tube which exhibits a linear increase in brightness, improved contrast and maintenance of color balance for all values of beam current.

Unitedv States Patent [72] lnventor Frank C. Palilla Maspeth, NY. [21] Appl. No. 677,198 [22] Filed Oct. 23, 1967 [45] Patented May 18, 1971 [73] Assignee General Telephone 8: Electronics Laboratories Incorporated [541 POSTDEFLECIION TUBE WITH ALL RARE EARTH PHOSPIIORS 2 Claims, 4 Drawing Figs.

[52] US. Cl. 313/92, 252/301.4 [51] Int. Cl ..H01j 29/32, H0 1 j 31/20 [50] field ofSearch 252/301.4; 313/92 (PH) [56] References Cited UNITED STATES PATENTS 2,169,046 8/1939 Headrick 313/92Pl-I 3,243,625 3/1966 Levine et a1. 3l3/92P1-1 3,393,336 7/1968 De France et a1. 313/92PDF 3,481,884 12/1969 Palila et a1. 252/301.4P

FOREIGN PATENTS 657,718 6/1965 Belgium 313/92PH 1,022,930 3/1966 Great Britain 31 3/92PH OTHER REFERENCES Nazarova, CATHODOLUMINESCENCE OF EUROPI- UM ACTIVATED STRONTIUM PHOSPHATES; Bulletin, Academy of Science, U.S.S.R; Vol. 25, 1961; pages 322, 323, 324 cited (252-301 .4?)

Primary ExaminerRobert Segal Attorneys-Robert J. Frank and Joseph H. Roediger AESI'A 1 CT: A color cathode-jay tube:oi the 'bbsideflciidh focusing type (PDF) wherein the phosphor screen exhibits a linear change in brightness with changes in electron beam current density over the entire operating range of the tube. The preferred phosphor screen employs trivalent europium activated yttrium vanadate as the red-emitting phosphor, trivalent terbium activated yttrium phosphate as the greenemitting phosphor and divalent europium activated strontium chlorophosphate as the blue-emitting phosphor. The disclosed screen results in a PDF tube which exhibits a linear increase in brightness, improved contrast and maintenance of color balance for all values of beam current.

Patented Ma 18, 1971 @IRIGIBIRIG l l l l l l l'r BRIGHTNESS EURRENT DENSITY IN VEN TOR.

FRANK C. PALILLA BY 1%. 9. m AT ORNEY POSTDEFLECTION TUBE WITH ALL RARE EARTH PHOSPHORS BACKGROUND OF THE lNVENTlON This invention relates to cathodoluminescent phosphor screens and to high-brightness, high-contrast cathode-ray tubes employing such screens.

Cathode-ray tubes of the type used in color television receivers comprise one or more electron guns positioned within one end of an evacuated envelope and a phosphor screen located at the other end of the envelope. Conventionally, the screen consists of three sets of phosphor elements which emit red, blue and green light respectively when bombarded by electrons emanating from the electron guns. The light emitted by the phosphor elements combines to produce an image having a color corresponding to the color of the object whose image is being transmitted.

The intensity of the luminescence emitted by the screen is dependent upon the number of electrons per second striking each incremental phosphor area and upon the velocity of the electrons. lt is desirable that, with a given accelerating volt-- age, the electron density be sufficiently large to permit viewing of the phosphor screen under bright ambient light and that the contrast ratio be as high as possible. The term contrast ratio expresses the relationship between the light emitted from the screen which contributes to the image being displayed, and the background or spurious light which is independent of the desired image and detracts therefrom.

The most common type of color television tube in use today is the shadow mask version in which a perforated electrode is interposed between three electron guns and the screen in a unipotential drift space. One set of three phosphor elements is in registration with each aperture in the mask, the size and location of the apertures and the size and location of the phosphor areas being so chosen that the electrons from each of the three guns can only reach a phosphor element which emits light of a particular color. The shadow mask tube has relatively poor efi'iciency since approximately 85 percent of the electrons emitted by the guns are stopped by the mask. lncreasing the beam current to provide higher brightness is not etfective because this can result in heating of the mask and subsequent misregistration of beam landings.

Other types of color picture tubes are available which make more efficient use of the electron beam energy. In one such class of tubes (hereinafter termed postdeflection-focusing or PDF tubes), the unipotential drift space is replaced by a retarding field region created by a grid in close proximity to the screen. A focusing action is thereby provided in the grid screen region with a net reduction in beam diameter at the screen from that existing at the grid. Because of this focusing action, the grid may have a much coarser structure than the shadow mask resulting in more efficient electron beam transmission. Consequently, the number of electrons per second reaching each incremental area of the phosphor screen can be increased considerably over that possible with the shadow mask tube.

Despite the greater beam currents possible with PDF tubes, the phosphor screens available heretofore have not provided increases in brightness proportional to the increased beam current and have exhibited color variations with changes in current density. Further, the contrast ratio of the PDF display has been found inferior to that obtained with a shadow mask tube. This disappointing performance is due in large measure to interactions which occur between the electrons impinging on the screen and the nonlinear phosphors comprising the prior art screens. More particularly, in known cathodoluminescent screens for color television at least one of the three phosphors exhibits a nonlinear increase in brightness as the beam current density is increased. That is, after an initial linear increase in brightness, the increase in brightness becomes smaller for each incremental increase in beam current density and the phosphor is said to saturate.

Typical of phosphor combinations which exhibit this nonlinear saturating characteristic is the all-sulfide" screen disclosed in US. Pat. No. 2,991,383 granted July 4, l96l. In the all sulfide screen all three of the phosphors saturate at current densities within the operating range of the PDF tube. An improved screen disclosed in US. Pat. No. 3,243,625 granted March 29, I966 employs a rare-earth-activated vanadiumcontaining phosphor in lieu of the red-emitting sulfide phosphor. The rare earth phosphor in this screen has a substantially linear response; however, the blue and greenemitting phosphors in the commercially produced screens are usually of the nonlinear sulfide type. In order to overcome the disadvantage of these prior art tubes, l have invented a cathodoluminescent screen which, when used with a color television tube of the PDF type, exhibits a linear increase in brightness over the entire operating range of the tube, has improved contrast ratio and maintains good color balance for all values of beam currents.

SUMMARY OF THE INVENTION ln accordance with the present invention, there is provided a cathode-ray tube for displaying high-contrast highbrightness color images. The tube consists of an evacuated envelope, at least one electron gun mounted within the envelope and a cathodoluminescent screen comprising a plurality of electron-responsive phosphor elements arranged in a predetermined pattern on a substrate located within the envelope. The light emitted by each of the phosphors changes substantially linearly in brightness as generated by the electron gas striking the phosphor is varied over the operating range of the tube. The combination of the light emitted by each of the phosphors produces a color of constant hue and increasing brightness as the density of the electron beam striking each phosphor is increased proportionally. in addition, the effect on contrast ratio of secondary and backscatter electrons is considerably less than for the conventional color screen,

More specifically, the cathodoluminescent screen of my invention comprises selected red, green and blue-emitting phosphor systems.

The red-emitting phosphor component has a host selected from the group consisting of yttrium oxide Y O gadolinium oxide 0e 0,, lutetium oxide b1 0,, yttrium oxysulfide Y 0 S, sodium gadolinate NaGd0 yttrium vanadate YV0 gadolinium vanadate GdV0 lanthanum oxychloride LaOCl and lutetium vanadate LuV0 these hosts being activated by an element selected from the group consisting of trivalent europium Eu and trivalent samariam Sm.

The green-emitting phosphor component has a host selected from the group consisting of indium borate lnB0 lutetium phosphate LuP0.,, lanthanum oxychloride LaOCl, yttrium phosphate YP0 yttrium silicate Y SiO and zinc silicate Zn Si0 The rare earth activators trivalent terbium TB, trivalent holmium Ho and trivalent erbium Er may be combined with the lnB0 LuP0 La0C1, YPO, and Y Si0 hosts. Divalent manganese Mn is employed as the activator for Zn SiO The blue-emitting phosphor component is selected from the group consisting of (l members of the alkaline earth haloborate system having the formulation 3MO -5B O M C1 Eu (2) members of the alkaline earth halophosphate system having the formulation wM (PO4)2' yM' R2:Eu, where M and M are alkaline earth elements selected from the group consisting of strontium, bar

ium and calcium, R is an ion selected from the halogen group, ylw is between 1 and 3, and x is between 0.0025 and 0.10

gram atom per gram atom of the host cation (M+M), and (3) compositions having a host selected from the group consisting of Y O 0e 0,, Lu O Y 0 S, NaGd0 (\l0 GdV0 LaOCl and LuVo all of these hosts being activated by trivalent thulium Tm.

the density of the beam The phosphor combination I prefer comprises trivalent europium-activated yttrium vanadate as the red-emitting component, trivalent terbium-activated yttrium phosphate as the green-emitting phosphor and divalent europium-activated strontium chlorophosphate as the blue-emitting component. The brightness of each of these phosphors, either alone or in combination, increases linearly with beam current density over the entire operating range of the PDF tube, has acceptable decay characteristics and does not exhibit significant change in color of light output with change in the beam intensity.

Brief Description of the Drawings FIG. 1 is a schematic representation of a PDF cathode ray tube,

FIG. 2 is a perspective schematic diagram showing details of a portion of the cathodoluminescent screen and electrondeflecting means of FIG. 1,

FIG. 3 illustrates schematically the paths of backscatter electrons produced by impingement of an electron beam on the screen, and

FIG. 4 is a graph showing the effect of stray electrons on the tube contrast ratio for a prior art phosphor and for a phosphor of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, there is shown schematically a typical three-gun, postdeflection-focusing, cathode-ray tube for use in color television receivers. The tube comprises three electron guns 10, 11 and 12 located within one end of an evacuated envelope l3 and a phosphor screen 14 deposited on the inside surface of the other end of the envelope. An aluminum electron-permeable layer 15 is deposited on the rear or gun side of the phosphor screen 14 and maintained at a potential of approximately 20,000 volts with respect to the cathodes of the electron guns. A focusing grid 16, composed of a multiplicity of wires 17 stretched across a frame 18, is positioned adjacent layer 15 and spaced therefrom. The grid is maintained at a considerably lower potential than layer 15, on the order of 4,500 volts, with respect to the gun cathodes.

Phosphor screen 14 is composed of a plurality of groups of adjacent electron-responsive phosphor stripes. Each group consists of a red-emitting, a green-emitting and a blueemitting phosphor stripe, the stripes being arranged across the screen in the order RGBRGBRGB. Each group of three phosphor stripes is associated with one wire 17 of the focusing grid 16. The grid wires 17 are connected together thereby maintaining all of the wires at the same DC potential.

As indicated schematically in FIG. 2, electron guns 10, 11 and I2 generate electron beams 20, 21 and 22 respectively. The electron guns l-12 are slightly inclined to the axis perpendicular to the phosphor screen 14 and consequently the electron beams converge. The potential difference between grid 16 and the conducting electron-permeable layer provides a postdeflection-focusing field which deflects the electron beams causing each to strike a phosphor stripe which emits light of a predetennined color. Thus, in FIG. 2, beam 20 impinges only on the blue-emitting stripes of screen 14, beam 21 only on the green-emitting stripes and beam 22 only on the red-emitting stripes.

In another version of the PDF tube not shown, only one electron gun is provided and the phosphor stripes are arranged with one color stripe centered, as for example, in the order RGBGRGBG. Adjacent red and blue-emitting stripes are electron-optically centered behind adjacent grid wires. The grid wires in the single gun version are divided into two mutually insulated interlaced sets and the single beam switched from stripe to stripe by varying the potential on the two sets of grid wires. Additional information on these well-known PDF tubes may be obtained from the article The PDF Chromatron-A single or Multigun Tricolor Cathode-Ray Tube". by R. Dressler in the July 1953 issue of Proceedings of the IRE.

As is apparent from the above description of the PDF tube, the magnitude of the electron beam or beams whichexcite the phosphors must be directly controllable. The beam must then 1 be focused, and its path controlled suchthat the appropriate phosphor area is excited. Electrons from any source which do not fulfill either requirement, yet reach the screen with sufficient energy to excite the phosphor, degrade performance and can be classified as stray" electrons. Some of the more significant sources of stray electrons in a PDF tube are:

l. Field emission from the electron guns or other structures within the tube,

2. Overscanned electrons, in the form of primary or secondary back scattered electrons which react the screen by travelling between the tube wall and the switching grid structure,

3. Secondary or backscattered electrons from the grid wires or grid support structure,

4. Backscattered electrons from the screen, which because of the electric fields present, return to the screen with sufficient energy to cause phosphor excitation.

The first two sources of stray electrons, field emission and overscanning, can usually be minimized by careful tube design. The latter two sources, secondary or backscattered electrons from the grid structure and backscattered electrons from the screen are much more difficult to reduce. Ac-

cordingly, stray electrons from the grid and screen are present .in PDF tubes and tend to interact with the phosphor screen to produce unwanted light output thereby reducing the contrast ratro.

The normal result of electron impact upon a metal surface, such as the grid wires 17, grid support frame 18 or aluminum screen backing I5, is the release of electrons of energy levels up to the incident beam energy. In the case of primary beam impact on grid wires 17 or support structures 18, most of the released electrons directed toward the screen 14 arrive at the screen with sufficient energy to cause .phosphor excitation. Primary beam impact at the screen produces, as indicated in FIG. 3 for one of the three electron beams, a similar release of electrons. These electrons, represented by dashed line 23, are subjected to a retarding electric field which causes some electrons to return to the screen after having followed a parabolic trajectory in the space between screen 14 and grid wires 17. The average energy with which stray electrons impinge on the phosphor screen is considerably less than the energy in the primary beam 21 and the electrons are distributed over a wider screen area. Hence, the density of the electrons reaching the screen due to secondary and backscatter effects is much lower than that in the primary beam 21.

FIG. 4 illustrates the effect of stray electrons on the contrast ratios of a screen employing prior art nonlinear saturating phosphors such as silver-activated zinc sulfide or zinc-cadmium sulfide, and on a screen employing linear phosphors. The density of the electron beam under normal operating conditions is represented by J and the density of the stray backscatter and secondary electrons striking the screen as 1,. If the brightness of the screen due to the electron beam is B, and that due to the stray electrons is B, then the contrast ratio may be defined as B,,-B,/B,. Since B is small compared to B the contrast ratio may be approximated as B /B,

Curve 25 illustrates the relationship between brightness and current density for a typical nonlinear saturating cathodoluminescent phosphor. The contrast ratio for this phosphor is approximately B,,,,/B,,,. Curve 26 depicts the relationship between brightness and current density for a linear phosphor comprising the screen of the present invention. The brightness of the linear material at low values of current density is less than for the nonlinear phosphor reducing its usefulness in shadow mask tubes employing relatively low-density electron beams. However, at current densities above the crossover value 1,, the brightness is greater than for the nonlinear phosphor. The contrast ratio for the linear phosphor is approximately B,,/B,, as indicated in FIG. 4. Since B,,,, B,, and B,,, B,, it can be seen that the contrast ratio H /B for the linear phosphor is greater than the contrast ratio B /B, for the nonlinear phosphor.

In a preferred embodiment of the invention, the tricolor cathodoluminescent screen 14 comprises europium-activated .fi sstsfsrr d r e Phosphor I activator yttrium vanadate as the red-emitting component, terbium-ac tivated yttrium phosphate as the green-emitting component and divalent europium-activated halogenated strontium phosphate as the blue-emitting component. A PDF tube was constructed in which these phosphors were deposited in vertical stripes across the width of the faceplate and a thin aluminum layer evaporated on the gun side of the stripes. The: tube was adjusted to give a white field at low current densities and the gun current then increased in steps to the maximum} operating current of the tube. It was found that there was no.

the beam on the phosphor was varied by means of an externally applied magnetic field. The brightness of the light. emitted by the phosphor screen under these conditions was substantially constant (within 1 percent) as the beam current density was adjusted from a value below 0.1 ampere per square centimeter to above 1.0 ampere per square centimeter.

The preferred activator concentration for the yttriumj vanadate is about 5 mole percent europium, for yttrium phosphate about 10 mole percent terbium and for strontium chlorophosphate about 2 mole percent divalent europium. However, activator ranges of 2 to 10 mole percent Eu for the red-emittingphosphor, 2.5 to mole percent Tb for the green-emitting phosphor and 0.25 to 10 mole percent Eu for the blue-emitting phosphor are within operative limits. Methods of preparing the preferred red-emitting phosphor and additional information concerning it may be found in U.S. Pat. application Ser. No. 334,565 tiled Dec. 30, 1963, now abandoned, the greememitting phosphor in copending U.S. application Ser. No. 653,669 filed July 17, 1967 now U.S. Pat. No. 3,481,884 granted Dec. 2, 1969 and the blue-emitting phosphor in copending continuation-impart U.S. application Ser. No. 785,406 filed June 25, 1968. The following table summarizes the characteristics of the phosphors comprising CIE color coordinates Deca (mlll seconds to Activator concentration (mole Emission percent) color a' Red 10 Green 2 Blue of the light emitted by the phosphor as the area of impact of 20 as so Summarizing, l have invented a cathodoluminescent screen W increases substantially linearly with increased current density over the entire operating range of the tube and there is no observable shift in color. As a result, the contrast ratio of the tube is high, greater brightness is obtained at high current densities than in previous tubes and good color balance is main tained over the entire tube-operating range. While my cathodoluminescent screen has been described in connection with a PDF tube, it should be understood that it may also be emp oyee-wit ther yp 9 995 1599325 31.-. a

As many changes could be made in the above construction and many different embodiments could be made without departing from the scope thereof, it is intended that all matter; contained in the above description or shown in the accom-;

panying drawings shall be interpreted as illustrative and not in a limiting sense.

focusing for displaying highcontrast, high-brightness color b. at least one electron gun mounted within said evacuated A envelqae at the other end thereof,

c. a phosphor screen comprising red-emitting, greenemitting and blue-emitting electron-responsive phosphor elements affixed to a substrate within said envelope as. stripes on said faceplate said red-emitting phosphor elements comprising yttrium vanadate activated by approximately 5 mole percent trivalent europium, said greenemitting phosphor elements comprising yttrium phosphate activated by between approximately 10 mole percent trivalent terbium and said blue-emitting phosphor elements comprising strontium chlorophosphate activated by approximately 2 mole pers t irsls tsstqpis and a focusing grid comprising a multiplicity of wires located within said envelope adjacent said phosphor screen, said wires being in registration with selected phosphor stripes said grid being maintained atan average potential below ths q sa P L'EEhQEFF.9FE1.

' I 2. A cathode-ray tube defined by claim 1 wherein three 

2. A cathode-ray tube defined by claim 1 wherein three electron guns are mounted within said evacuated envelope and all of the wires comprisIng said focusing grid are coupled together thereby maintaining them at the same potential with respect to said phosphor screen. 